We will study the vasodilatory regulation of the cerebral and coronary circulation in human subjects. The effects of Nitric Oxide Synthase inhibitor L-NMMA on the cerebrovascular response to hypercapnia and on the response of the coronary circulation to cold pressor test will be studied in 20 healthy adults. Transcranial doppler with CO2 response (cerebral vasomotor reactivity and pulse doppler with M-Mode Echo for changes in coronary sinus will be used for this purpose.